In recent years, it has become increasingly necessary to rehabilitate the superstructures of highway bridges. In order to properly rehabilitate the superstructures of various constructions, decisions must be made concerning the adequacy of the existing foundations. This is particularly true for older structures for which as-built records are missing and for which foundation deterioration may have occurred. Visual inspection of foundations is virtually impossible. As such, a need has developed so as to provide procedures for evaluating the capacity of existing foundations. In particular, a need has developed to provide a procedure for determining the length of a pile in such foundations. A "pile" is defined as a member with a small cross-sectional area (in comparison with its length) used to provide adequate support for a column or wall resting on soil which is too weak or too compressible to support the structure with a spread footer. As used herein, the term "bent" includes piers or other structures above the foundation.
There is a serious need to rehabilitate the aging highway system. Within the United States, over 35% of the 575,410 bridges in the 1992 National Bridge Inventory were classified as needing to be replaced or rehabilitated. It has been estimated that over the next 20 years approximately $165 billion must be invested to address the tremendous rehabilitation backlog and to improve accruing bridge deficiencies. The economic value of the foundations for many bridges can be up to 25% of the cost of the bridge. This makes foundations a major economic component in the rehabilitation/repair effort. Inadequate foundations can, of course, jeopardize the entire superstructure of any rehabilitated bridges.
Bridge safety issues are foremost among the considerations for rehabilitation. Foundation failures, or excessive foundation movements, mostly from the application of extreme event loads, have occurred too frequently in recent years, exposing the public to risks that can be reduced by evaluation of the adequacy of existing foundations. Examples of major fatal catastrophes are the Sunshine Skyway Bridge in Florida (35 deaths), the Schoharie Creek Bridge in New York (15 deaths), the collapse of the Nimitz Freeway viaduct during the Loma Prieta earthquake (67 deaths), and a barge impact of a bridge in New Orleans (1 death). Clearly, upgrading of structures without appropriate knowledge of the adequacy of the foundations increases this vulnerability.
The traditional approach for evaluating such structures is to examine the as-built records. The as-built records include information on number, depth and width of the foundation elements, the soil characteristics at the bridge site and the recorded observations of the inspector during construction (concerning the potential for structural defects in the foundations). If necessary, as-built conditions can be confirmed by probing the exterior of the foundation and/or coring concrete piles or drilled shafts, if appropriate equipment can be positioned for the task. Once the loads for the rehabilitated structure are known, the capacity of these foundations can be evaluated in light of modern geotechnic design methods and the adequacy of the foundation determined. In the event that serious difficulties were noted during the installation of one or more foundation elements, or if probes and cores reveal defects, judgment must be exercised whether to exclude the questionable foundation element from consideration as a load-bearing pile or shaft. On occasions where the superstructure load can be taken off the foundation during the rehabilitation process, representative piles or shafts can be subjected to load tests, which is the most definitive way to evaluate the adequacy of the foundation. New geometrically identical "sister" piles or shafts can be installed immediately adjacent to the foundation of interest and subjected to load tests.
If as-built records are not available, or if they are incomplete, the traditional approach is not always appropriate because destructive probing and coring necessary to completely identify the foundation must be very extensive. In this case, nondestructive testing methods can be employed. Appropriate types of nondestructive tests for this purpose include pulse-echo and impulse-response testing, steady-state vibration testing, ambient vibration surveys, and shear-wave seismic reflection profiling. Various other techniques, such as casting sensors, or access tubes for sensors, into the pile or shaft, are not practical for evaluating existing foundations.
The pulse-echo and impulse-responsive testing involves the application of low-amplitude, impulse-type elastic waves directly to the head of an element of the foundation (pile or drilled shaft) with measurement of the reflected compression waves (P-waves) or shear waves (S-waves) from the bottom of the element or from a significant defect within the element, if such a defect exists. The input signal (load time history from an impulse source, for example a hammer, that creates the elastic wave in the foundation) can be measured along with the reflected signal (velocity or acceleration time history on the element near its top), or only the strain time history signal at the head of the pile may be measured, and the data processed in several ways. If a time history graph of the strain signal is displayed, peaks in the signal of a sensor on the element at known times can sometimes be interpreted as representing points of reflection if the compression or shear wave velocity of the pile material can be estimated. This so-called "pulse-echo" method has been applied mostly to piles and drilled shafts that are directly accessible (so that instruments can be attached directly to the pile or shaft and not to a cap, bent, column, or abutment) and has been applied to the investigation of both structural integrity and as-built depths of foundations.
Impulse-response testing (sometimes referred to as transient response testing) can be used for the same purpose, although it is somewhat more complex. With this method, both the input (elastic impulse source) and output (sensor) signals are recorded and processed in the frequency domain by a computer to develop a "mobility" function, which varies with frequency. In an ideal foundation, the mobility-frequency diagram makes it much more straightforward to interpret depths to major defects or pile/shaft lengths where defects do not occur. However, the presence of multiple defects in the foundation, reverberations from the superstructure, and other factors make this method difficult to use in evaluating existing foundations.
A disadvantage of the pulse-echo and impulse-response (mobility function) tests are that they appear to require that the sensor be placed on the foundation element (pile or shaft) itself, which may be difficult in some bridge foundations.
A well-established method for the characterization of the dynamic behavior of a structure is the steady-state vibration test. The superstructure is excited by a mechanism that generates a steady sinusoidal force in time. After a short period of time, the structure settles into a periodic steady-state mode of response at the exciting frequency. The force generator can be a small electromagnetic device, a mechanical device with a pair of counter-rotating masses, or a large mass driven by a linear actuator. The structural response can be monitored by displacement, velocity or acceleration sensors. By exciting the structure at several frequencies, a frequency response curve is obtained for a given point on the structure, from which modal frequencies and damping ratios are derived. This method has been rather widely applied to buildings, bridges, nuclear power plant structures and dams. When applied to bridges, it is mainly used to study the vibration of the superstructure. This forced vibration method can conceivably be used to infer foundation performance at low strain levels, but it is not likely to be useful in this respect because the overall system response of the structure depends very little on the foundation contribution. That is, any foundation behavior is masked, perhaps totally, by the superstructure behavior and cannot be separated from the system response, as the entire superstructure-foundation system is responding to the single frequency of the exciting force.
An ambient vibration survey records the vibration of a structure caused by ambient forces, such as wind, microtremors, traffic or any other forms of excitation that tend to be random and sustained, but small in amplitude. This method is most useful for characterizing the overall behavior of the structure, and when applied to a bridge, it is again limited in terms of characterizing foundation response because the dominant response will be from the superstructure.
Another technique that has been utilized is a technique for imaging shear-wave diffractions from pile terminations. This technique was described in an article by Ebrom et al. as published in the Society of Exploration Geophysics Convention Abstracts, 1994. This method is used to determine the subsurface lengths of terminations for a shaft or pile. In this method, it is necessary to perform a shear-wave survey in the immediate proximity of the pier and to infer the depth from the two-way travel time. This survey is aimed at delineating a terminating vertical unit, such as the shaft or pile. The goal of this method is to enhance diffracted seismic waves from the base of the shaft or pile. These diffractions are created when the shear-wave seismic wave field encounters the abrupt termination of the shaft or pile. The diffraction event is proportional in amplitude to the incident wave and the shear modulus contrast between the soil and the shaft or pile. The diffractions from the terminus of the shaft or pile possessing large modulus contrasts are easily detectable. In a typical highway environment, the shear-wave modulus contrast between near-surface soils and concrete are quite large, generally far exceeding a factor of 10:1. In this method, a horizontal array of sound sensors is provided in an area surrounding the bent or pier. A horizontal or vertical hammer blow is applied to the bent or pier. The elastic wave sensors will receive the diffracted waves from the bottom of the shaft or pile so that calculations can be carried out as to the length of the shaft or pile. This method includes a Kirchhoff migration by summing together the amplitudes that lie along the diffraction hyperbola (as calculated from the velocity field of the medium), and placing the summed amplitudes at the apex of the hyperbola. The apex of the diffraction hyperbola corresponds geometrically to the position of the diffracting point. After migration, diffracting points are imaged as high-amplitude events. Unfortunately, this method is often difficult to apply in areas in which space is limited. If it is not possible to arrange a large horizontal array of sensors in a location adjacent to the bent or pier, then this method cannot be effectively used.
It is an object of the present invention to provide a method which effectively determines the length of a shaft or pile.
It is another object of the present invention to provide a method for determining the length of a shaft or pile which is non-destructive.
It is a further object of the present invention to provide a method for the determination of a shaft or pile which is easy to use, easy to implement, and relatively inexpensive.
It is a further object of the present invention to provide a method for the determination of the length of a shaft or pile which can be utilized in a relatively limited physical area.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.